1. Field of the Invention
The invention relates to a shift control device and a shift control method and a shift control method of a vehicular automatic transmission that establishes a plurality of gear steps of different gear ratios by selectively engaging a plurality of engagement elements. More particular, the invention relates to a technology of reducing the shift shock in a downshift (coast downshift) during a decelerating run (coasting run) of a vehicle.
2. Description of the Related Art
In relation to the vehicular automatic transmissions that establish a plurality of gear steps of different gear ratios by selectively engaging a plurality of engagement elements (engagement devices), there is a known technology for enabling a vehicle to accelerate in good response (responsiveness) by appropriate driving power when the vehicle is to be accelerated again from a decelerating state. In this technology, during a deceleration of the vehicle, a coast downshift is performed to prepare for depression of the accelerator pedal. Besides, in relation to a vehicular automatic transmission as mentioned above, Japanese Patent Application Publication No. JP-A-2003-269601 describes a control device of an automatic transmission in which when a coast downshift involving an engagement switch between a release-side friction engagement device and an engagement-side friction engagement device (clutch-to-clutch switch) is to be performed, a good shift characteristic is achieved by using a shift line provided for the coast downshift which is different from the shift lines for normal shifts.
However, in the above-described related art, in the case where a brake operation is performed during a shift, there is a possibility of failing to secure a control precision and therefore resulting in occurrence of a shift shock, because, for example, it may become difficult to engage the engagement-side engagement element at a predetermined synchronous timing depending on changes in the deceleration caused by the brake, or other incidents may occur. Therefore, a shift control device of a vehicular automatic transmission that reduces the shift shock while enabling the vehicle to accelerate in good response when the vehicle is to be accelerated again from a decelerating state has been demanded.
Generally, in the aforementioned clutch-to-clutch coast downshift, good control precision regarding the synchronous timing of engaging the engagement-side engagement device, or the like is required in order to reduce the shift shock involved in the coast downshift. However, in the case where a brake operation is performed during a coast downshift, there is a possibility of failing to secure a control precision and therefore resulting occurrence of a shift shock, because, for example, it may become difficult to engage the engagement-side engagement element at a predetermined synchronous timing depending on changes in the deceleration caused by the brake, or other incidents may occur.
To overcome this drawback, the following measure is conceivable. That is, if there is a driver's intention to decelerate the vehicle, such as a brake operation or the like, during a coast downshift, a shift wait control of stopping the rise of the engagement pressure of the engagement-side engagement device is performed. On the other hand, if there is no driver's intention to decelerate the vehicle during a coast downshift, the shift wait control is removed to cause the coast downshift to progress. In this manner, it may be possible to achieve both improved acceleration response at the time of re-acceleration from the decelerating state of the vehicle and reduced shift shock.
However, this measure has the following problem. That is, in the case where during the shift wait control, the input rotation speed of the automatic transmission has risen so that there is a great differential rotation speed (rotation speed difference) between the input rotation speed and the synchronous rotation speed for the shift-target gear step of the coast downshift, and where the driver's intention to decelerate the vehicle then disappears, so that the shift wait control is removed to cause the coast downshift to progress, there is possibility of occurrence of the problem of a large shift shock due to the large rotation speed difference.